The deployment, confirmed to The New Arab by Iraqi officials, coincides with a twin bombing targeting Shia pilgrims in the Syrian capital, later claimed by Sunni jihadists.
More than 600 fighters from the Nujaba movement, League of the Righteous and Imam Ali Battalions - all with direct links to Iran - were reportedly moved from the Iraqi city of Beiji to Damascus.
A video has emerged (below) purporting to show a military parade involving around 50 Shia fighters being drilled by Iranian and Syrian regime officers outside the Sayyida Zaynab Shrine. The video was uploaded last week, before the bombings.
Thousands of Iraqi Shia Muslim fighters are known to be fighting alongside the forces of the Assad regime, under the supervision of Iran's elite Qods Force and Lebanon's Shia militant group Hizballah.
The stated purpose of their intervention is protecting Shia shrines across the border, but their deployment has taken them far from that destination.
Around 5000 Iraqi militia fighters were dispatched to the front-lines of Aleppo in October to assist pro-regime forces recapture the city from rebel groups.
Militias to which the fighters are affiliated have been accused of taking part in large-scale human rights violations against civilians in Iraq, prompting calls for investigations and accountability.